Kog'Maw and Lulu have withstood the test of time as a phenomenal duo in the bot lane; let's break down why they are so good.
Kog'Maw is the quintessential late game hyper carry: his damage output can be incredible, but he has no mobility and is vulnerable when alone. In the laning phase, he can slow enemies with his E then trade with enhanced basic attacks by proccing his W. However, considering his Q passively grants bonus attack speed, Kog'Maw's power is in the late game.
Once Kog'Maw reaches the late game, he can shred through enemies by landing his Q to reduce their armour and MR. He can also play relatively safely in the backline by proccing his W to increase his basic attack range. His ultimate can be tricky to land, but can help finish off low health opponents thanks to its execute damage.
Meanwhile, Lulu is a lane bully. Her basic attacks, E and Q can whittle away at enemies in the laning phase, and help establish lane control. Given the fact W increases movement and attack speed on allies or can CC enemies, it's best used as a defensive tool in the early game. It should be saved for when the enemy bot lane engages, or the enemy jungler comes in for a gank.
Lulu's ultimate is pretty diverse; it can be used to save an ally who is on low health, or be used as a secondary engage tool for allies who are in the middle of multiple enemies.
Lulu allows Kog'Maw to output even more damage and play more aggressively. When Lulu uses her W and E on Kog, it will
- increase his damage
- increase his attack speed, allowing him to fire more auto attacks that are empowered by his W and Lulu's E
- increase his movement speed, meaning he can continue to damage enemies who try to run away
Lulu's W shouldn't always be used on Kog though, as it should be used to polymorph enemies that are trying to kill Kog. This hard CC is often more valuable than granting movement speed when you're trying to keep Kog alive. Lulu's ultimate can also be used to save the Kog if he is vulnerable.
With Lulu offering Kog a lot more protection, Kog can play a lot more forward when Lulu is around. Once Kog has a couple of items, it is possible to run straight at enemies and decimate them with empowered basic attacks. If any enemies try and fight back, Lulu can CC them and ult Kog while he kills them.
Given how much Kog'Maw stacks attack speed, Lethal Tempo is a great keystone as you can exceed the attack speed cap at max stacks. Kog can also afford to take some tankier runes like Conditioning and Overgrowth without sacrificing his damage, allowing him to play more aggressively without dying.
For items, Kog wants to build attack speed and on-hit items like Guinsoo's Rageblade, Runaan's Hurricane, and Wit's End. As mentioned, Kog can build tankier and buy Randuin's Omen to prevent getting blown up and slow enemies that get too close.
Lulu's runes are pretty flexible. Guardian provides another layer of protection for your allies (including Kog), but Aery is a great rune for increasing your poke damage in lane while still shielding allies. You can switch between Revitalize and Unflinching should the enemy have a lot of CC, or Bone Plating and Second Wind if the enemy has all-in or poke damage.
Shurelya's Battlesong is generally the best Mythic for Lulu, speeding up allies whenever you use your W or E on them. This means it has an extremely high uptime over the course of a game. Lulu will also build standard enchanter items, such as Redemption, Ardent Censer and Chemtech Putrifier.
Kog'Maw and Lulu are all about scaling. The duo can become unstoppable if Kog can get ahead, as Lulu can keep increasing his damage and protect him from any incoming threats.